It is known to treat reticulated water with chlorine and ammonia in order to produce chloramines, principally monochloramine. Monochloramine is used as a disinfectant within the water. Monochloramine is produced according to the following reactions:Cl2+H2O⇄HOCl+HClNH3+HOCL⇄NH2Cl+H2O
The ration of chlorine:ammonia-nitrogen in the above reactions is approximately 5:1. The reactions may take place in the presence of excess aqueous ammonia, for instance in a concentration of about 0.2 mg/L.
One problem which arises in the use of chloramines in water is that the chloramines have a propensity to decay. This decay can be a simple chemical decay, causing the equilibrium in the second equation above to move to the left. This chemical decay is very slow, and free ammonia is formed as chlorine levels are being reduced. Tests have revealed that this decay can reach extremely high levels, due to nitrification of ammonia. Nitrification occurs when ammonia is oxidised to nitrite by microbiological organisms, either within the water or on an inner surface of pipes. The nitrite is then further oxidised to nitrate by other organisms.
The microbiological organisms generally form a biofilm. Once a biofilim is established, nitrification can continue, even in the presence of high levels of monochloramine (levels greater than 2 mg/L, expressed as chlorine).
The most common method of ameliorating this problem is to periodically change the disinfectant within the reticulation system from chloramines to free chlorine. This is achieved by the introduction of chlorine into the water. This has the effect of destroying the biofilm.
Several problems are associated with this method. These include the presence in the newly chlorinated water of an undesirable level of disinfectant by-products, in addition to a noticeable change in the taste and smell of the water. Should the quantity of added chlorine not be sufficient, it can actually reduce the effective chlorine in the water, resulting in inadequate disinfection.
Further, where water is being reticulated over a long distance, a periodic change from chloramination disinfection to free chlorine disinfection requires the presence of significant treating infrastructure at locations along the distance.
Other methods have been proposed in order to ameliorate the problem of nitrification. They include the addition of chlorite ions and also significantly increasing pH. None of these solutions have thus far proved satisfactory.
Consequently, it is currently necessary to retreat chloraminated water being reticulated over long distances at various stages along the reticulation path. This requires the addition of farther chlorine and/or ammonia to bring the amount of chloramines or chlorine in the water back to effective levels.
The present invention attempts to overcome at least in part some of the aforementioned propensity of chloramines in treated water to decay.